Vibration isolators may be designed to isolate vibration produced by a vibrating element supported on a support structure from a substrate or to isolate an element (such as a work surface or enclosure) supported on a support structure from a vibrating substrate. Conventional vibration isolation structures for both these applications generally have fixed isolation characteristics and are designed to isolate vibration optimally at the predominant or average excitation frequency occurring in the system. However, most systems experience variations in excitation frequency of the vibration source and variations in structural characteristics that affect the stiffness, mass, or mass distribution in the system. Changes in the excitation frequency of a system may occur, for example, due to speed changes in process pump/motor sets operating on variable frequency drives or speed changes in marine propulsion units to accommodate schedule and sea conditions. Changes in structural characteristics of a system may occur, for example, due to changes in a tank level, piping, or ship cargo mass and distribution.
Vibration isolators with fixed isolation characteristics have proven to be a significant problem in applications in which minimum structural or acoustic vibration is important, such as, for example, high tech fabrication facilities, marine propulsion systems, and refrigeration units near customers in retail food sales/service. Vibration isolators with fixed isolation characteristics generally are designed to have optimal transmissibility (i.e., the ratio of the amplitude of the force transmitted to the support structure to the amplitude of the exciting force of the vibration source) at the predominant or average excitation frequency occurring in the system. The transmissibility of a vibration isolator with fixed isolation characteristics varies with respect to the excitation frequency of the vibration source. Therefore, even slow variations in semi-steady state excitation frequency or structural characteristics result in unacceptable excitation of the system when using an isolator with fixed isolation characteristics.
Accordingly, there is a need in the art for a vibration isolator with an adjustable response that provides optimal vibration isolation despite changes in excitation frequency of the vibration source or changes in the structural characteristics of the support structure. Preferably, such vibration isolators are useful both for isolating a vibration source from a substrate and for isolating a work surface from a vibrating substrate.